The field of this invention is that of electrical circuit breakers and the invention relates more particularly to thermally responsive circuit breakers for interrupting electrical circuits on the occurrence of predetermined conditions in the circuits.
Thermally responsive electrical circuit breakers typically interrupt electrical circuits in response to the occurrence of selected overload currents in the circuits to protect other equipment in the circuits from damage due to overheating or the like. In one particularly advantageous breaker shown in U.S. Pat. No. 3,361,882 commonly assigned to assignee of the present invention, a control mechanism manually moves movable contacts into and out of engagement with complementary contacts to open and close a circuit and a thermally responsive bimetallic member is operable to open the circuit in response to the occurrence of a selected overload current in the circuit. The bimetallic member is formed of metal materials having substantial electrical resistance properties and the member is disposed in the breaker circuit so that the member is self-heated and flexes to a selected extent to trip the mechanism to open the breaker circuit when a selected overload current flows in the circuit for a selected period of time. That circuit breaker has a high quality reliable structure and is adapted to be latched in open circuit position until manually reset after it has been manually opened or has been opened in response to the occurrence of an overload current. It is also "trip free" in that the circuit breaker will open in response to the occurrence of an overload current even if the manual resetting means is manually held in the circuit closing position.
However, for some applications it would be highly desirable for such a circuit breaker to be characterized by additional backup features to assure fail safe operation and to open the breaker circuit on the occurrence of an overload condition even if the described control mechanism should fail to operate or should be otherwise ineffective to separate the breaker contacts. It would also be desirable to achieve such fail safe operation at low cost without reducing the sturdiness and reliability of the breaker and while permitting the breaker to incorporate other known features and operating characteristics.